Fisher Nicholas, Kramer David M
MSU-DOE Plant Research Laboratory, Michigan State University, East Lansing, MI 48824, USA.
MSU-DOE Plant Research Laboratory, Michigan State University, East Lansing, MI 48824, USA; Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, MI 48824, USA.
Biochim Biophys Acta. 2014 Dec;1837(12):1944-1954. doi: 10.1016/j.bbabio.2014.09.005.
Non-photochemical (dark) increases in chlorophyll a fluorescence yield associated with non-photochemical reduction of redox carriers (Fnpr) have been attributed to the reduction of plastoquinone (PQ) related to cyclic electron flow (CEF) around photosystem I. In vivo, this rise in fluorescence is associated with activity of the chloroplast plastoquinone reductase (plastid
NAD(P)H: plastoquinone oxidoreductase) complex. In contrast, this signal measured in isolated thylakoids has been attributed to the activity of the protein gradient regulation-5 (PGR5)/PGR5-like (PGRL1)-associated CEF pathway. Here, we report a systematic experimentation on the origin of Fnpr in isolated thylakoids. Addition of NADPH and ferredoxin to isolated spinach thylakoids resulted in the reduction of the PQ pool, but neither its kinetics nor its inhibitor sensitivities matched those of Fnpr. Notably, Fnpr was more rapid than PQ reduction, and completely insensitive to inhibitors of the PSII QB site and oxygen evolving complex as well as inhibitors of the cytochrome b6f complex. We thus conclude that Fnpr in isolated thylakoids is not a result of redox equilibrium with bulk PQ. Redox titrations and fluorescence emission spectra imply that Fnpr is dependent on the reduction of a low potential redox component (Em about − 340 mV) within photosystem II (PSII), and is likely related to earlier observations of low potential variants of QA within a subpopulation of PSII that is directly reducible by ferredoxin. The implications of these results for our understanding of CEF and other photosynthetic processes are discussed.
与氧化还原载体的非光化学还原相关的叶绿素a荧光产量的非光化学（暗）增加（Fnpr）被归因于与围绕光系统I的循环电子流（CEF）相关的质体醌（PQ）的还原。在体内，这种荧光增加与叶绿体质体醌还原酶（质体NAD(P)H：质体醌氧化还原酶）复合物的活性有关。相比之下，在分离的类囊体中测量到的该信号被归因于与蛋白质梯度调节-5（PGR5）/PGR5样（PGRL1）相关的CEF途径的活性。在此，我们报告了关于分离类囊体中Fnpr起源的系统实验。向分离的菠菜类囊体中添加NADPH和铁氧还蛋白导致PQ库的还原，但其动力学和抑制剂敏感性均与Fnpr不匹配。值得注意的是，Fnpr比PQ还原更快，并且对PSII的QB位点和放氧复合体的抑制剂以及细胞色素b6f复合体的抑制剂完全不敏感。因此，我们得出结论，分离类囊体中的Fnpr不是与大量PQ的氧化还原平衡的结果。氧化还原滴定和荧光发射光谱表明，Fnpr依赖于光系统II（PSII）内低电位氧化还原成分（Em约为-340 mV）的还原，并且可能与早期观察到的PSII亚群中可被铁氧还蛋白直接还原的QA低电位变体有关。讨论了这些结果对我们理解CEF和其他光合过程的意义。
